A known image scanner reads a document by moving, along a sub-scanning direction, a line sensor extending in a main scanning direction. The image scanner detects the presence or absence of a document on a document table by comparing between data read at a specified position with respect to the main scanning direction and a threshold determined based on color strength of white document background. In response to the line sensor reaching a trailing edge of the document while moving along the sub-scanning direction, the color strength of data read at the specified position indicates the color strength other than the color strength of the white document background. In response to such color strength change, the image scanner stops reading the document.